The present invention relates to a liquid crystal display device, and especially relates to a liquid crystal display device comprising liquid crystal regions divided by polymer walls and having axially symmetrically aligned liquid crystal molecules within said liquid crystal regions.
Heretofore, a TN (twisted nematic)-type liquid crystal display or an STN (super twisted nematic)-type liquid crystal display utilizing nematic liquid crystal materials is known as a display device utilizing electrooptical effect.
In order to widen the viewing angle of the liquid crystal display, for example, Japanese Patent Application Laid-Open Publication Nos. 6-301015 and 7-120728 disclose a liquid crystal display having liquid crystal molecules axially symmetrically aligned within each of a plurality of liquid crystal regions divided by a polymer wall, or so-called ASM (axially symmetrically aligned microcell)-mode liquid crystal display device (prior art example 1). The liquid crystal regions each substantially surrounded by a polymer wall are typically formed corresponding to each pixel element. Since the liquid crystal molecules are axially symmetrically aligned in the ASM-mode liquid crystal display device, the viewer can observe the liquid crystal display device from any direction without the contrast being varied greatly, or in other words, the ASM-mode display device has a wide viewing angle characteristic.
The ASM-mode liquid crystal display device disclosed in the above publications is manufactured by providing polymerize-induction-phase-separation to the mixture including a polymerized material and a liquid crystal material.
The method for manufacturing the liquid crystal display device according to prior art example 1 is explained in detail with reference to FIG. 10. First, as shown in FIG. 10(a), a substrate is prepared having color filters and electrodes (not shown) formed on one surface of a glass substrate 11xe2x80x2 (step a).
Next, as shown in FIG. 10(b), on the surface of the glass substrate 11xe2x80x2 equipped with the electrodes and color filters, a polymer wall 13xe2x80x2 for axially symmetrically aligning the liquid crystal molecules is formed, for example in a lattice-shape (step b). Actually, after spin-coating a photosensitive resin material on the surface of the glass substrate 11xe2x80x2 equipped with the color filters and electrodes, the material is exposed through a photo-mask having a predetermined pattern and then developed, in order to form a lattice-shaped polymer wall 13xe2x80x2. The photosensitive material utilized in this step could either be negative or positive. According to another example, the polymer wall may also be formed using a resin material without photosensitivity. In that case, however, a step for forming a separate resist layer must be added to the process.
Next, as shown in FIG. 10(c), by exposing/developing the photosensitive resin material, pillar-like protrusions 17xe2x80x2 are separately formed, through patterning, on some areas of the top of the polymer wall 13xe2x80x2 selectively (step c).
Next, as shown in FIG. 10(d), the surface of the glass substrate equipped with the polymer wall 13xe2x80x2 and the pillar-like protrusions 17xe2x80x2 is covered with a vertical alignment film 18xe2x80x2 made for example of polyimide (step d).
On the other hand, the surface of an opposing glass substrate 21xe2x80x2 shown in FIG. 10(e) equippedwith electrodes (not shown) is covered with a vertical alignment film 28xe2x80x2 (step f).
Next, as shown in FIG. 10(g), the two substrates 11xe2x80x2 and 21xe2x80x2 are bonded together with the surfaces equipped with electrodes facing each other, in order to form a liquid crystal cell (step g). The gap between the two substrates is defined by the sum of the heights of the polymer wall 13xe2x80x2 and the pillar-like protrusion 17xe2x80x2. Therefore, the thickness of the liquid crystal layer (cell gap) could be adjusted to a preferred value.
Next, as shown in FIG. 10(h), liquid crystal material 32xe2x80x2 is injected by a vacuum injection method and the like, to the liquid crystal region 31xe2x80x2 formed within the liquid crystal cell (step h).
Lastly, as shown in FIG. 10(i), voltage is applied to the pair of electrodes arranged in opposed positions, in order to axially symmetrically align the liquid crystal molecules 32xe2x80x2 within the liquid crystal region 31xe2x80x2 (step i). The liquid crystal molecules within each of the liquid crystal regions divided by the polymer wall 13xe2x80x2 are axially symmetrically aligned with a center axis 33xe2x80x2 shown by a broken line that is perpendicular to the pair of substrates.
The cross-sectional structure of a conventional color filter 12xe2x80x2 is shown in FIG. 11. The color filter 12xe2x80x2 includes a black matrix (BM) 13xe2x80x2 for blocking the gap formed between colored patterns, and a colored resin layer 14xe2x80x2 colored to red, green and blue (R, G, B) corresponding to each pixel element, which are formed on the glass substrate 11xe2x80x2. An overcoat (OC) layer 19xe2x80x2 formed of acrylic resin, epoxy resin and the like having a thickness of approximately 0.5-2.0 xcexcm is formed on the colored resin layer, so as to improve the smoothness of the surface. On top of the overcoat layer is formed a transparent electrode 15xe2x80x2 formed of indium tin oxide (ITO) film. The BM film 13xe2x80x2 is typically formed of a metal chromium film having a thickness of approximately 100-150 nm. The colored resin layer 14xe2x80x2 is formed by coloring a resin material with dyestuff or pigment, with a typical thickness of approximately 1-3 xcexcm.
In order to form the color filter, a colored resin layer having photosensitivity formed on a substrate is patterned through photolithography method. For example, three photosensitive resin materials each colored to red (R), green (G) or blue (B) are utilized, and through coating/exposing/developing of each colored photosensitive resin material, a color filter having R, G, B colors is manufactured. The method for forming the photosensitive colored resin layer includes applying the liquid-state photosensitive colored resin material diluted by a solvent onto the substrate by a spin-coating method and the like, or transferring the photosensitive colored resin material in dry-film form onto the substrate. By providing a color filter to the ASM-mode liquid crystal display device, a colored liquid crystal display device having a wide viewing angle characteristic could be obtained.
However, the ASM-mode liquid crystal display device and the method for manufacturing the same according to the above-mentioned prior art example 1 have the following problems. That is, according to the prior art liquid crystal display device, there is a need to form a separate polymer wall in order to axially symmetrically align the liquid crystal molecules. The existence of such polymer wall increases the resist existing when injecting the liquid crystal material, and as a result, increases the injection time greatly. Therefore, in order to reduce the resist during liquid crystal material injection, the height of the polymer wall is minimized. However, in such case, the height of the pillar-like protrusion made of resin material formed to define the thickness of the cell has to be relatively increased. Especially, when manufacturing a large-scale liquid crystal display device, it is difficult to form pillar-like protrusions made of a thick-film resin having the same height throughout the whole area of the large-sized substrate. As a result, the thickness of the cell becomes uneven within the display region of the liquid crystal display device, which causes unevenness of the brightness, the color, and the viewing angle characteristic of the display, deteriorating the display quality.
The present invention aims at solving the problems of the conventional method and device. The object of the present invention is to provide a liquid crystal display device having improved display quality and wide viewing angle characteristic, capable of preventing unevenness of cell gap within the display region, and reducing problems caused by such uneven cell gap, such as uneven brightness, color and viewing angle characteristic.
The present invention provides a liquid crystal display device comprising a pair of substrates, and a liquid crystal layer formed between said pair of substrates: wherein one of said pair of substrates is equipped with a first polymer spacer, and a second polymer spacer having portions overlapping the first polymer spacer, with pillar-like protrusions formed on the overlapping portions of said second polymer spacer.
Further, the present invention provides a liquid crystal display device, wherein the first polymer spacer is stripe-shaped.
Further, the present invention provides a liquid crystal display device, wherein the second polymer spacer is formed of transparent resin.
Moreover, the present invention provides a liquid crystal display device, wherein the second polymer spacer is stripe-shaped.
The present invention provides a liquid crystal display device, wherein the second polymer spacer is positioned substantially orthogonal to the first polymer spacer.
The present invention provides a liquid crystal display device, wherein the second polymer spacer is lattice-shaped.
Further, the present invention provides a liquid crystal display device, wherein the height of the second polymer spacer is either equal to or greater than the height of the first polymer spacer.
Moreover, the present invention provides a liquid crystal display device, wherein the thickness of the liquid crystal layer is defined by the sum of the height of the first polymer spacer, the height of the second polymer spacer, and the height of the pillar-like protrusion.
The present invention provides a liquid crystal display device, wherein the first polymer spacer consists of the laminated portion of a shading layer and a colored layer partially overlapping said shading layer.
Even further, the present invention provides a liquid crystal display device, wherein the first polymer spacer and the second polymer spacer divides the liquid crystal layer into plural liquid crystal regions, and axially symmetrically align liquid crystal molecules within each liquid crystal region.
The operation of the liquid crystal display device according to the present invention will now be explained. In the case where the liquid-state photosensitive resin material is applied on the substrate, uneven film thickness is caused mainly by two reasons. One cause is related to the method for applying the resin material. For example, according to the spin-coating method, uneven film thickness is generated either radially or concentrically. The other cause of uneven thickness is related to the resin material itself, such as the surface tension of the resin material, or the uneven volatility of the solvent. This type of unevenness does not depend on the application method, and the shape of the unevenness is similar (scally, for example).
After minimizing the uneven film thickness caused by the application method by controlling the condition of application and the like, the uneven film thickness caused by the material itself (the second cause of unevenness) becomes elicit.
The unevenness of film thickness becomes more obvious as the thickness of the formed film becomes greater. Especially, when using the photosensitive resin material as the spacer for determining the liquid crystal cell gap, the unevenness of the film thickness is the main cause that deteriorates display quality. In order to obtain a good display quality, it is necessary to reduce the unevenness of the film thickness as much as possible.
Especially, in a large-scale liquid crystal display device, not only the unevenness of display having a large pitch caused by the difference between the maximum value and the minimum value of the film thickness that effects the whole display surface, but also the unevenness of display caused by fine unevenness of film thickness that is generated by a finer pitch (with a distance of approximately few pixel elements to a few hundred pixel elements) is also noticeably observed. The display unevenness having a large pitch is mainly caused by the unevenness of film thickness caused by the application method, and the unevenness of film thickness with a finer pitch is often caused by the material itself.
It is discovered that in order to effectively reduce the unevenness of film thickness with a finer pitch caused by the material, the resin material should be applied by multiple layers through plural steps instead of applying all the material at once, and to form a resin material having a desired thickness by the multilayers. When the material is applied through a number of steps, the film thickness to be formed in a single step is reduced, and as a result, the unevenness of film thickness formed by a single step is also reduced. When utilizing a liquid-phase material, the unevenness of film thickness is not simply added according to the number of layers being laminated. Therefore, the unevenness of film thickness when the multiple layers finally reach the desired thickness is much smaller than when the material is applied all at once. This is because each layer of film applied on top of the other layer of film acts to fill up the unevenness of the layer formed beneath. Actually, in case the unevenness caused by applying the film to a certain thickness is xc2x15% the film thickness, the unevenness of film thickness having a thickness of 6 xcexcm is xc2x10.3 xcexcm. However, when the film is applied through three steps to form the same thickness, each step applying the film to a thickness of 2 xcexcm, then only the unevenness caused by the final layer having a thickness of 2 xcexcm is observed, and the unevenness of the film becomes approximately xc2x10.1 xcexcm.
Therefore, by forming the spacer defining the cell gap by multiple layers, the unevenness of film thickness is reduced, and as a result, the display quality is improved.
However, according to the method of applying the material through plural steps without performing a patterning step each time to the layer, and instead, performing the patterning after all the layers are formed, it is discovered that the material applied in multiple layers effect each other, and the unevenness of the layer thickness is not sufficiently reduced. In other words, when the second layer is applied on top of the first layer that has been applied and pre-baked, the solvent included in the material of the second layer affects the material of the first layer, and creates an even larger unevenness of film thickness.
Therefore, in order to form a multi-layer film pattern, it is necessary to perform patterning and post-baking to every layer, so that the solvent included in the next layer does not affect the layer underneath.
In order to apply the above method to the conventional spacer or pillar-like protrusion defining the cell gap of the prior art ASM-mode liquid crystal display, it is necessary to divide the step for forming the pillar-like protrusion into multiple steps. In order to do so, the steps for applying and patterning the material is increased according to the number of the multiple layers, and this increases the manufacturing cost as a result.
The present invention aims at reducing the unevenness of film thickness without increasing the number of steps involved in manufacturing the film. Therefore, according to the present invention, the pillar-like protrusions are formed on the crossing points of the step-shaped structure formed to the color filter and the transparent structure for axially symmetrically aligning the liquid crystal molecules. By applying such structure, the present invention provides a liquid crystal display having an even cell gap, the effect of which is equal to forming the spacer defining the cell gap through multiple steps. Accordingly, the present invention provides a liquid crystal display including a multi-layered spacer defining the cell gap, without increasing the number of manufacturing steps compared to the conventional ASM-mode liquid crystal display device. As a result, the present invention realizes a liquid crystal display having a good display quality with reduced cell gap.